HI!
Thanks for advice.
Now I'm wondering Q1. Where comes its collector current?
I draw the schematic so that positive side is upper and it's now easier to understand, but Q1 seems to be quite weird on ordinary differential amp.

DJK you are right, there is nothing wrong with the design, it is just not drawn nicely that is all. The person who drew the schematic forgot to put a ground connection between the 4700uF power supply caps.